Approximately thirty minutes into the flight, the hijackers forcibly breached the cockpit and overpowered the pilot and first officer, allowing lead hijacker and trained pilot Marwan al-Shehhi to take over the controls. Unlike Flight 11, which turned its transponder off, the aircraft's transponder was visible on New York Center's radar, and the aircraft deviated from the assigned flight path for four minutes before air traffic controllers noticed these changes at 08:51 EDT. They made several unsuccessful attempts to contact the cockpit. Unknown to the hijackers, several passengers and crew aboard made phone calls from the plane to loved ones and provided information about the hijackers and injuries to passengers and crew.

The aircraft crashed into Tower Two (the South Tower) of the World Trade Center at 09:03. The Flight 175 hijacking was coordinated with that of American Airlines Flight 11, which struck the top of Tower One (the North Tower) 17 minutes earlier. The crash of Flight 175 into the South Tower was the only impact seen live on television around the world as it happened. The impact and subsequent fire caused the South Tower to collapse 56 minutes after the crash, resulting in hundreds of additional casualties. During the recovery effort at the World Trade Center site, workers recovered and identified remains from Flight 175 victims (see the Aftermath section below), but many other body fragments could not be identified.

In early September 2001, the Flight 175 group of hijackers arrived in Boston from Florida. Hamza al-Ghamdi and Ahmed al-Ghamdi arrived together on September 7 and checked into the Charles Hotel in Cambridge, Massachusetts. The next day, they relocated to the Days Inn in Boston. Fayez Banihammad flew from Florida to Boston, along with Mohand al-Shehri, on September 8, and they checked into the Milner Hotel in Boston. Marwan al-Shehhi arrived in Boston on September 9 and stayed at the Milner Hotel, where he shared a room with Flight 11 hijacker Mohamed Atta.[3]

The flight was operated with a Boeing 767-200, registration number N612UA, built and delivered in 1983,[4] with capacity of 168 passengers. It had 168 seats (10 in first class, 32 in business class, and 126 in economy class). On the day of the attacks, the flight carried only 56 passengers and 9 crew members, which represented a 33 percent load factor — well below the average load factor of 49 percent in the three months preceding September 11.[5] The nine crew members included Captain Victor Saracini, First Officer Michael Horrocks, and flight attendants Robert Fangman, Amy Jarret, Amy King, Kathryn Laborie, Alfred Marchand, Michael Tarrou, and Alicia Titus.[6] Excluding the hijackers, the passengers on the flight included 35 men, 12 women, and three children who were all under the age of 5.[7]

Hamza al-Ghamdi and Ahmed al-Ghamdi checked out of their hotel and called a taxi to take them to Logan International Airport.[8] They arrived at the United Airlines counter in Terminal C at 06:20 Eastern Time and Ahmed al-Ghamdi checked in two bags. Both hijackers indicated they wanted to purchase tickets, though they already had paper tickets. They had trouble answering the standard security questions, so the counter agent repeated the questions very slowly until the men gave the correct answers.[2][9] Hijacker pilot Marwan al-Shehhi checked in a single bag at 06:45, and the other remaining hijackers, Fayez Banihammad and Mohand al-Shehri, checked in at 06:53. Banihammad checked two bags.[2] None of the hijackers were selected for extra scrutiny by the Computer Assisted Passenger Prescreening System (CAPPS).[10]

Shehhi and the other hijackers boarded Flight 175 between 07:23 and 07:28. Banihammad boarded first and sat in first class seat 2A, while Mohand al-Shehri was in seat 2B. At 07:27, Shehhi and Ahmed al-Ghamdi boarded, and sat in business class seats 6C and 9D respectively. A minute later, Hamza al-Ghamdi boarded, and sat in 9C.[2][10]

The flight was scheduled to depart at 08:00 for Los Angeles. Fifty-one passengers and the five hijackers boarded the 767 through Terminal C's Gate 19. The plane pushed back at 07:58 and took off at 08:14 from runway 9,[2][11] about the same time Flight 11 was hijacked. By 08:33, the aircraft reached cruising altitude of 31,000 feet, which is the point when cabin service would normally begin.[2] At 08:37, air traffic controllers asked the pilots of Flight 175 whether they could see American Airlines Flight 11. The crew responded that Flight 11 was at 29,000 feet, and controllers ordered Flight 175 to turn and avoid the aircraft.[12] The pilots declared that they had heard a suspicious transmission from Flight 11 upon takeoff. "Sounds like someone keyed the mic and said everyone stay in your seats," the flight crew reported. This was the last transmission from Flight 175.[13]

It is estimated that Flight 175 was hijacked between 08:42 and 08:46, while Flight 11 was just minutes away from hitting the North Tower.[2] According to Flight 175: As the World Watched, it's believed that "muscle hijackers" Fayez Banihammad and Mohand al-Shehri forcibly entered the cockpit and killed the pilots while Hamza al-Ghamdi and Ahmed al-Ghamdi started moving passengers and crew to the back of the aircraft. The first operational evidence that something was abnormal on Flight 175 came at 08:47, when the plane's transponder signal changed twice within the span of one minute, and the aircraft began deviating from its assigned course.[11][14] However, the air traffic controller in charge of the flight did not notice until minutes later at 08:51.[2] Unlike Flight 11, which had turned its transponder off, Flight 175's flight data could still be properly monitored.[14] Also, at 08:51, Flight 175 changed altitude. Over the next three minutes, the controller made five unsuccessful attempts to contact Flight 175 and worked to move other aircraft in the vicinity away from Flight 175.[2]

At around this time, the flight had a near midair collision with Delta Air Lines Flight 2315, reportedly missing the plane by only 300 feet, as air traffic controller Dave Bottiglia frantically tried to tell the Delta pilot to take evasive action. Bottiglia was the first person in the control center to realize that Flight 175 was hijacked when he gave directions for a turn. Flight 175 did not respond, instead accelerating and heading toward the Delta plane. The controller commanded the Delta pilot, "Take any evasive action necessary. We have an airplane that we don't know what he's doing. Any action at all."[15][16] Moments before Flight 175 crashed, it avoided another near collision with Midwest Express Flight 7, which was flying from Milwaukee to New York.[17]

At 08:55, a supervisor at the New York Air Traffic Control Center notified the center's operations manager of the Flight 175 hijacking, and David Bottiglia, who was handling both Flight 11 and Flight 175, noted, "we might have a hijack over here, two of them."[2] At 08:58, the plane was over New Jersey at 28,500 feet, heading toward New York City. In the five minutes from approximately 08:58 when Shehhi completed the final turn toward New York City until the moment of impact, the plane was in a sustained power dive, descending more than 24,000 feet in 5 minutes 4 seconds, for an average rate of over 5,000 feet per minute.[14] New York Center air traffic controller Dave Bottiglia reported he and his colleagues "were counting down the altitudes, and they were descending, right at the end, at 10,000 feet per minute. That is absolutely unheard of for a commercial jet."[16]

The complete 9/11 Commission Report available from the archived version of the 9/11 Commission website.

Flight attendant Robert Fangman, as well as two passengers (Peter Hanson and Brian David Sweeney), made phone calls from GTE airphones in the rear of the aircraft. Airphone records also indicate that Garnet Bailey made four phone call attempts, trying to reach his wife.[18][19]

Fangman called a United Airlines office in San Francisco at 08:52, and spoke with Marc Policastro. Fangman reported the hijacking and said that the hijackers were likely flying the plane. He also said that both pilots were dead and that a flight attendant was stabbed.[11] After a minute and 15 seconds, Fangman's call was disconnected.[18] Policastro subsequently made attempts to contact the aircraft's cockpit using the Aircraft Communication Addressing and Reporting System (ACARS) message system.[2]

Brian David Sweeney tried calling his wife, Julie, at 08:58, but ended up leaving a message, telling her that the plane had been hijacked. He then called his parents at 9:00 a.m. and spoke with his mother, Louise. Sweeney told his mother about the hijacking and mentioned that passengers were considering storming the cockpit and taking control of the aircraft.[11]

At 08:52, Peter Hanson called his father, Lee Hanson, in Easton, Connecticut, telling him of the hijacking. Hanson was traveling with his wife, Sue, and 2½-year-old daughter, Christine, who had never flown on a plane before. The family was originally seated in Row 19, in seats C, D, and E; however, Peter placed the call to his father from seat 30E. Speaking softly, Hanson said that the hijackers had commandeered the cockpit, that a flight attendant had been stabbed, and that possibly someone else in the front of the aircraft had been killed. He also said that the plane was flying erratically. Hanson asked his father to contact United Airlines, but Lee could not get through and instead called the police.[20][21]

Peter Hanson made a second phone call to his father at 09:00:

“

It's getting bad, Dad. A stewardess was stabbed. They seem to have knives and Mace. They said they have a bomb. It's getting very bad on the plane. The plane is making jerky movements. I don't think the pilot is flying the plane. I think we are going down. I think they intend to go to Chicago or someplace and fly into a building. Don't worry, Dad. If it happens, it'll be very fast...Oh my God...oh my God, oh my God."[16]

”

As the call abruptly ended, Hanson's father heard a woman screaming.[16]

At 09:01, two minutes before impact as Flight 175 continued its descent into Lower Manhattan, the New York Center alerted another nearby Air Traffic Facility responsible for low-flying aircraft, which was able to monitor the aircraft's path over New Jersey, and then over Staten Island and Upper New York Bay in its final moments.[14] (Flight 175 came in from the southwest, apparently heading for the Empire State Building, but turned right, then left into the South Tower.)

At exactly 9:03:02, Flight 175 slammed nose-first into the southern facade of South Tower of the World Trade Center, at a speed of approximately 590 mph (950 km/h, 264 m/s, or 513 knots)[22] and striking between floors 77 and 85 with approximately 10,000 U.S. gallons (38,000 L; 8,300 imp gal) of jet fuel on board.[14][23] The youngest person on Flight 175 was 2½-year-old Christine Hanson of Groton, Massachusetts, and the oldest was 80-year-old Dorothy DeAraujo of Long Beach, California. Hundreds more were killed within the tower and from its ensuing explosion, fires, and eventual collapse. It is estimated that 637 people were killed instantly or trapped at and above the floors of impact in the South Tower.

Based on the position of the aircraft from eyewitness statements and video footage, the aircraft was in a banking left turn in its final moments, as it appeared that the plane might have otherwise missed the building or merely scraped it with its wing. Upon crashing, the plane was banked left. Those who were on the left side of the plane would, therefore, have had a clear view of the towers approaching, with one burning, until the final moment of the flight.[16]

By the time Flight 175 struck the South Tower, multiple media organizations were already covering the crash of Flight 11, which had hit the North Tower 17 minutes earlier. The image of Flight 175's crash was thus caught on video from multiple vantage points on live television and amateur video, while approximately 100 cameras captured Flight 175 in photographs before it crashed.[24] Video footage of the crash was replayed numerous times in news broadcasts on the day of the attacks and in the following days, before major news networks put restrictions on use of the footage.[25]

After the plane penetrated through the tower, part of the plane's landing gear and fuselage came out the north side of the tower and crashed through the roof and two of the floors of 45–47 Park Place, between West Broadway and Church Street, (600 feet (180 meters)) north of the former World Trade Center. The plane parts destroyed three-floor beams and severely compromised the building's internal structure.[22][26][27][28]

Unlike at the North Tower, initially, one of the three stairwells was still intact after Flight 175 crashed into the South Tower. This was because the plane struck the tower offset from the center and not centrally as Flight 11 in the North Tower had done. Only 18 people passed the impact zone through the available stairway and left the South Tower safely before it collapsed. One of them, Stanley Praimnath, was on the 81st floor, and his office suffered a direct hit. He witnessed Flight 175 coming toward him.[16] One of the wings sliced through his office and wound up wedged in a doorway approximately 20 feet away from him.

Some people above the impact zone made their way upward toward the roof in hopes of a helicopter rescue. However, access doors to the roof were locked. In any case, thick smoke and intense heat prevented rescue helicopters from landing.

Some debris from the aircraft were recovered nearby, including landing gear found on top of a building on the corner of West Broadway and Park Place, an engine found at Church & Murray Street, and a section of the fuselage landed on top of 5 World Trade Center.

During the recovery process, small fragments were identified from some passengers on Flight 175, including a six-inch piece of bone belonging to Peter Hanson,[29] and small bone fragments of Lisa Frost.[30] Remains of many others aboard Flight 175 were never recovered.[31]

Shortly after September 11, the flight number for future flights on the same route was changed from Flight 175 to Flight 1525 "out of respect for those who died in the attack."[32] Since then, United Airlines has renumbered and rescheduled all flights from Boston to Los Angeles, and none of its morning flights depart at 8:00 AM EDT.[33] As of August 2012, this flight was redesignated Flight 1494, now departing at 8:34 AM EDT, using a Boeing 737-800. It was reported in May 2011 that United was reactivating flight numbers 175 and 93 as a codeshare operated by Continental, sparking an outcry from some in the media and the labor union representing United pilots.[34][35][36] However, United said the reactivation was a mistake and said the numbers were "inadvertently reinstated," and would not be reactivated.[35]

On April 26, 2013, a piece of the inboard wing flap mechanism from a Boeing 767[37] was discovered wedged between two buildings at Park Place, near where landing gear parts were found.[38]